So… we know that Luke didn’t both take the weapon AND kill the Emperor so we have to jump into the else block. Assuming that Luke’s side did not change, and he started out Light, then he would still be light.

Then the next thing: the false assumption that hate makes you powerful. This claim is not substantiated. Luke asks Yoda about this straight up in the Empire Strikes Back.

Excerpt

Luke: (…) Is the dark side stronger?
Yoda: No, no, no. Quicker, easier, more seductive.
Luke: But how am I to know the good side from the bad?
Yoda: You will know… when you are calm, at peace, passive. A Jedi uses the Force for knowledge and defense, NEVER for attack.

The author tries to make the claim (as does the Emperor) that hatred leads to power and that Luke feels it coarsing through him. However, we know, as did Luke, that the dark side is not stronger.

I’d like to think that Luke is having an internal struggle between both sides. It is at this crucial point that he remembers his conversation with Yoda back on Dagobah. When it comes down to it, how will he know the good side from the bad side? The answer, as you read above, is that you will know when you are calm, at peace, and passive.

The author of this article carefully leaves out the bit just after Luke stares at his hand and supposedly claims power. Let me fill it in. Luke calmly collects himself, turns off his Lightsaber, throws it away, and then addresses the Emperor.

So… the very last actions that Luke takes are ones that a “good” Jedi would take, given our definition of what makes a “good” Jedi.

Here, Luke clearly chooses to be calm, peaceful, and passive – the Emperors plans to turn him have failed. Obviously, the Emperor would then be a bit miffed, but to say that he now viewed Luke as a rival is taking it a bit too far. If Luke had truly turned, do you honestly think he would’ve turned off his lightsaber? No… he would’ve used the force to attack the Emperor and strike him down.

Remember: Luke does not strike the Emperor down, so his journey to the darkside is not complete. Here, in the moment where he could’ve claimed evil victory he chooses not to.

In the moment of his greatest temptation Luke chose Light side qualities. Thus, I can say with confidence, that Luke Skywalker is a Jedi Knight.